This invention relates to multi-speed drives for screen printing presses.
In screen printing, as is known, an ink is transferred to the printing surface by means of a transfer member or squeegee through a stencil screen. The basic operation requires a flood stroke followed by a print stroke. In the flood mode, when the flood stroke is made, a flood bar is caused to move across the screen to flood the screen with ink, and in the print mode, when the print stroke is made, a squeegee then moves across the screen to push the ink through the screen onto the object being printed.
Uniform ink distribution during the flood mode is not only highly desirable, but essential to obtaning high-quality prints. In order to obtain such uniform ink distribution, it has been found that the operating speed at which the flood bar is caused to move across the screen during the flood mode should be somewhat slower than the operating speed at which the squeegee moves across the screen during the print mode. Further, the inks utilized on the screen may vary in viscosity and consistency depending on job requirements, atmospheric conditions and other factors; and therefore, the speed at which the flood bar should move across the screen to optimally flood the screen can vary from ink to ink and object to object.
Generally, in the prior art, the print and flood strokes were coordinated to operate at the same speed, that of the flood stroke, which normally was the lower of the two speeds. Thus, the output of the press was limited to the speed at which the screen could be flooded, even though a higher printing speed, if possible, would be desirable. Attempts to achieve this separate speed ability in the flood mode and the print mode utilized separate sources of power to drive the squeegee and the flood bar during their respective strokes.
Note that silk screen printing processes involve various types of relative displacements between the surface to be printed, the screen, and the squeegee dependent on the printing surface. In the case of a flat continuous printing surface, such as paper stock, the screen and the printing surface may be held relatively stationary with respect to each other and the squeegee and the flood bar moved across the screen. In the case of printing surfaces which are not flat, the screen may be held stationary and the work surface subjected to rotational movement; or, the screen may be moved in synchronism with the curved surface. Thus, it is a purpose and object of the present invention to provide means which are universal for adjusting and controlling the operating speed of movement of the squeegee across the associated screen; and for adjusting and controlling the speed of movement of the flood bar at a second speed, independently of the squeegee and regardless of the shape of the object to be printed upon.
An advantage of the invention is that the operator can adjust either the printing or flooding speed independently to suit the characteristics of the ink being used. This is particularly useful in printing a heavy bodied or viscous ink which requires a slower than normal flood speed but a relatively normal print speed. An example of a somewhat different requirement, that also benefits from independent adjustment of the speed of the flood and print strokes, arises when printing with vinyl inks since vinyl inks require a particularly fast print stroke for good definition, but a much slower flood stroke.